A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging the pattern onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to control the lithographic process to place device features accurately on the substrate, alignment marks are generally provided on the substrate, and the lithographic apparatus includes one or more alignment measurement systems by which positions of alignment marks on a substrate can be measured accurately. These alignment measurement systems are effectively position measuring apparatuses. The alignment marks aid in the accurate placement of a process layer formed on the substrate relative to previously formed process layers. Various different types of alignment marks and different types of alignment measurement systems are known. Generally, an alignment measurement system measures the position of an alignment mark by irradiating it with a measurement radiation beam, receiving at least a portion of the measurement radiation beam scattered from the alignment mark and determining a position of the alignment mark from this scattered radiation. Alignment measurements are typically made, within a lithographic apparatus, each time a substrate is loaded into the lithographic apparatus, before each process layer is formed.
Once two or more process layers have been formed on a substrate, it may be desirable to measure how accurately the different process layers are aligned. Any shift or offset of one process layer relative to another may be referred to as an overlay and may adversely affect the integrated circuit (if for example the overlay is above a threshold tolerance). In order to measure overlay, each process layer may be provided with one or more overlay mark. The overlay marks may each comprise one or more reflective grating. Such overlay measurements are typically made once both process layers have been formed, for example outside of the lithographic apparatus.
There is continually a need to provide more accurate position measurements, especially to control overlay as product features get smaller and smaller.
As an integrated circuit is fabricated on a silicon wafer, the alignment marks can be buried by various layers of the integrated circuit. The thicknesses and optical properties of these layers can vary according to the type of integrated circuit. One or many of these layers can be opaque and, as a result, a measurement radiation beam may not be able to penetrate through the layers and reach the alignment mark. This represents a big obstacle for state of the art optical alignment and overlay methods.
It is an object of the present invention to provide alternative methods and apparatus that are suitable for determining overlay which at least partially address one or more problems associated with prior art arrangements, whether identified here or not.